1. Field
The subject invention is in the general field of tools and implements used in the manufacture and construction of buildings. More specifically it is in the field of implements used in the installation of dry walls in such buildings. Still more specifically it is in the field of flat finishing boxes, particularly the heads used on flat finishing boxes. The heads incorporate a flexible blade, often termed the edge or the trowelling bar of the head, and the edge is adjustable to contour the mastic (mud) distributed from the head in a contour ranging from convex (i.e. arced away from the plane of the drywall panels) to flat to concave (i.e. arced into the plane of the drywall panels). The subject invention involves the edge and the edge adjustment mechanism.
2. Prior Art
The closest known prior art to the subject invention is shown in U.S. Pat. Nos. 2,984,857 (Ames) and 4,516,818 (Molnar). In the major embodiment of the Ames apparatus the trowelling bar of the head of the mastic applicator (flat finishing head) is held in a flexible guide strip. The guide strip is held at its ends so that is may be adjustably arched to arch the trowelling bar to influence the curvature of the surface of the mastic (mud) distributed from the box. The apparatus described thus far is common to the subject invention and the two noted prior art concepts. Ames' invention lies in his apparatus for adjusting the amount and direction (i.e. upward or downward) arching of the guide strip and with it the trowelling bar. In the Ames adjustment apparatus the guide bar is arched in its free state and the adjustment apparatus forces it to decrease the arch. The Ames adjustment apparatus contacts the guide bar at the two locations essentially midway between its center and its ends. This is an advantage of the Ames apparatus because it enables adjusting the bar to be essentially straight to produce a flat surface on the dispensed mud. A disadvantage of the Ames apparatus is that the guide strip must be carefully, precisely arched during manufacture and this is known to be difficult to do.
The Molnar apparatus has the advantage of using a guide strip which is straight in its free state and therefore less expensive to manufacture with more predictable use results. However, the Molnar guide strip is connected to the guide strip adjustment apparatus at the center point of the strip. This is a disadvantage in that when the strip and bar are adjusted to be arched the arch is not an arc but is, instead, two arcs and this is not an optimum result.
Accordingly, the object of the subject invention is to provide adjustment mechanism for arching the guide strip and trowelling bar which has the stated advantages of the Ames and Molnar applications and does not have their stated disadvantages. In other words, the objective is to provide an adjustment mechanism which functions with a guide strip which is straight in its free state and contacts the guide strip at two points essentially midway between its center and its ends, i.e. at its quarter points.